UV Mapping - Boolean through a plane
3D software needs to interpret faces with 3 or 4 vertices. Everything else is called n-gon. In this case you have an n-gon with around 26 vertices. Select this face, hit x, delete just this face. than select two vertices in corners (drawing), press F to create edges between them, than fill everything out. With 3 faces this should be solved. You can also try to press K (knife tool) to draw the missing edges.